1. Field of the Invention
The present invention relates to a method of manufacturing a driving-device for a unit pixel of an organic light emitting display, and more particularly, to a method of manufacturing a driving-device for a unit pixel of an organic light emitting display having an improved manufacturing process in which the driving device can be manufactured with a smaller number of processes and in simpler processes.
2. Description of the Related Art
Liquid crystal displays (LCDs) which are light and have low power consumption, have been most noticed in the field of flat panel displays (FPDs). However, since LCDs are light-receiving type displays, there are technical restrictions in brightness, contrast, view angle, and large size or the like. Development for new FPDs that can overcome these disadvantages has briskly proceeded.
Organic light emitting displays, which are one of new FPDs, have a two-dimensional arrangement of unit organic light emitting devices (OLEDs) and are a self light-emitting type. Thus, organic light emitting displays have excellent characteristics, such as excellent view angle and contrast and do not need backlights. Thus, organic light emitting displays can be made lighter and smaller and are advantageous to power consumption. In addition, since organic light emitting displays can be directly driven by low voltages, have a fast response speed and are entirely solid bodies, the organic light emitting displays are strong to an external shock and have a wide usage temperature range, and in particular, have low manufacturing costs. In particular, if organic light emitting displays are driven using an active matrix method by which transistors as switching devices are disposed in each pixel, the organic light emitting displays show the same brightness even if low currents are applied to the organic light emitting displays. Thus, the organic light emitting displays have low power consumption and high definition and can be made large.
Two thin film transistors (TFTs), that is, a switching TFT and a driving TFT, must be provided in each unit pixel of the active matrix type organic light emitting displays, so as to drive an OLED. The switching TFT turns on/off a current supplied to each unit pixel, and the driving TFT provides currents allocated to the OLED. Due to this role assignment, characteristics and specifications required for the switching TFT and the driving TFT are different. For example, the switching TFT must have a small leakage current and a large on/off ratio. On the other hand, the driving TFT is required to have high mobility and reliability. In order to meet the requirements, the switching TFT needs to be formed in a structure including an amorphous silicon channel region or a polycrystalline silicon channel region having a very small grain size, and contrary to this, the driving TFT needs to be formed in a structure including a polycrystalline silicon channel region having a comparatively large grain size.
However, to this end, a process of manufacturing the switching TFT and a process of manufacturing the driving TFT must be separately performed. Thus, there are problems that manufacturing costs increase and manufacturing processes are complicated. Meanwhile, when an amorphous silicon TFT is simultaneously used as a switching TFT and a driving TFT, the amorphous silicon TFT is worthy of a switching TFT but has low electron mobility and reliability to be used as a driving transistor. Thus, it is difficult to implement an OLED having excellent characteristics. Contrary to this, when a polycrystalline silicon TFT is used as a switching TFT, the polycrystalline silicon TFT is worthy of a driving TFT but has a large leakage current in an off-state to be used as a switching transistor. Thus, the function of the polycrystalline silicon TFT as a switching transistor is lowered and it is difficult to implement an OLED having excellent characteristics.